#include <bits/stdc++.h>
using namespace std;
int main() {
    int n, ans = 0;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; ++i)
        cin >> a[i];
    for (int i = 1; i < n - 1; ++i)
        if ((a[i - 1] < a[i] && a[i] > a[i + 1]) || (a[i - 1] > a[i] && a[i] < a[i + 1]))
            ++ans;
    cout << ans;
    return 0;
}
